Floating captive screw

ABSTRACT

A captive screw for securing together two panels has a screw and a spring extending between the screw head and a ferrule. A collar is formed on the screw shank under the ferrule to limit penetration of the screw threads in a bottom panel so that the upper panel floats on the spring.

[0001] This application claims the benefit of U.S. patent application Ser. No. 09/803,221 filed Mar. 9, 2001, entitled floating captive screw which is based on U.S. Provisional Application No. 60/188,406 filed Mar. 10, 2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to captive screws.

[0004] 2. Brief Description of the Prior Art

[0005] Captive screws are screws that are “captivated” or held within a collar or ferrule. The ferrule is mounted in a first panel or like object. The screw portion is captivated so that it can be moved perpendicular to the first panel and the screw's threads can engage a corresponding aperture in a second panel, so that the first and second panels can be secured together. However, when the panels are unscrewed and disengage, the screw is retained in the first panel. Captive screws are useful in applications where it is important to avoid dropping or losing screws during assembly or repair, such as electronic devices, where a lost screw can cause catastrophic electrical shorts damaging equipment.

[0006] One particular application for captive screws is to hold down a heat sink which is mounted on top of a microchip and is intended to dissipate the heat of the microchip. The microchip, in turn, is fastened to a circuit board. Oftentimes, between the heat sink and the microchip a compressible or elastic material in the form of the sheet is placed. If one were to put the heat sink directly on top of the microchip, there would be tiny gaps between the two. Since air conducts heat poorly, these gaps have a detrimental effect on heat transfer. This interface material in the form of a sheet is needed and the sheet has a high thermal conductivity. Recently, microchips during operation have been getting warmer and warmer as space has been getting more and more limited on circuit boards. Accordingly, heat sinks have been getting larger and larger as the requirements for withstanding vibrations has been getting more stringent. Standoffs have been used which rise from the board to which the heat sinks are screwed down. Difficulties encountered with standoffs have been that the standoffs do not compensate for chip height variation and they do not provide consistent compression of the interface material between the chip and the heat sink.

[0007] In the past, various clamps, latches, pins, etc have been used to hold heat sinks down. Plastic clips have been used, however, this requires that the use of a screwdriver to screw in the clip directly onto the circuit board which makes the board susceptible to damage. Plastic clips, although they are not clostly, are limited by the weight of the heat sink and the clips have proven to be unreliable in vibration and drop shock testing. Some varieties of clips are clipped onto the heat sink at the ends of fins in the middle of the heat sink. Unfortunately, the middle fins are the most important fins in a heat sink as they carry most of the heat load. Therefore, when such clips are used the need for a larger heat sink results.

[0008] Accordingly, a need exists for a captive screw which is captive or fixed to a first panel such as a heat sink whereby a collar on the screw bottoms out at a fixed distance on a lower panel when the screw is screwed into the lower panel thereby allowing the captive screw to provide a constant pressure on the top panel. In addition, the captive screw should be able to pass stringent vibration testing and be able to allow for thermal expansion.

[0009] The present invention has been developed in view of the foregoing and to overcome the deficiencies of the prior art.

SUMMARY OF THE INVENTION

[0010] The present invention solves the problem of providing for secure fastening of a lower panel to an upper panel while providing a constant pressure on an upper panel to which the captive screw is captive.

[0011] In one embodiment of the present invention, a captive screw is disclosed which includes a ferrule, a screw, and a spring. The screw has a head, a shank adapted to pass through the ferrule, a threaded portion at the end of the shank opposite the head, and a collar formed on the shank proximate the threaded portion. The screw is captivated on the ferrule between the head and the collar. The spring extends on the shank of the screw between the head and the ferrule. Preferably, the ferrule has a generally cylindrical exterior surface having a plurality of knurls for securing the captive screw in a preformed aperture in a first structure, such as a lever. Is also preferred that the ferrule be formed with an inner ring formed on the interior surface of the ferrule having a generally circular opening large enough to permit the threads collar the screw to pass through but not large enough to permit the spring to pass through.

[0012] In a second embodiment of this invention, the ferrule has an annular lip and an exterior circumferential exterior groove. Preferably, the annular lip and exterior groove permit the captive screw to be press fit in a structure.

[0013] In a third embodiment of the present invention, the ferrule of the captive screw has a first and a second annular lip which provide for the captive screw to be pressed into a structure.

[0014] In a fourth embodiment of the present invention, a captive screw is provided which has a collar connected to the shank of the screw in the proximity of the threaded portion of the screw. The collar has outwardly extending legs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a side elevational view of a first embodiment of a captive screw according to the present invention, partially in section in a heat sink and a circuit board.

[0016]FIG. 2 is an exploded perspective view of a first embodiment of the screw of the captive screw of FIG. 1.

[0017]FIG. 3 is a side elevational cross-sectional view of the captive screw of FIG. 1 in an unfastened position.

[0018]FIG. 4 is a side elevational cross-sectional view of the captive screw of FIG. 1 in a fastened position.

[0019]FIG. 5 is a side view of the captive screw of FIG. 1 in a fastened position.

[0020]FIG. 6 is an isometric view of the captive screw of FIG. 1 in a fastened position.

[0021]FIG. 7 is a side view of the captive screw of FIG. 1 in an unfastened position.

[0022]FIG. 8 is a perspective view of the captive screw of FIG. 1 in an unfastened position.

[0023]FIG. 9 is a top plan view of the captive screw of FIG. 1.

[0024]FIG. 10 is a side view of the screw in the captive screw of FIG. 1.

[0025]FIG. 11 is a perspective view of the screw in the captive screw of FIG. 1.

[0026]FIG. 12 is a perspective view of the spring in the captive screw of FIG. 1.

[0027]FIG. 13 is a top view of the ferrule in the captive screw of FIG. 1.

[0028]FIG. 14 is a side view of the ferrule in the captive screw of FIG. 1.

[0029]FIG. 15 is a perspective view of the ferrule in the captive screw of FIG. 1.

[0030]FIG. 16 is a side elevational view of a second embodiment of the captive screw of the present invention partially in section.

[0031]FIG. 17 is a side view of a third embodiment of the captive screw of the present invention.

[0032]FIG. 18 is a side elevational view of the captive screw of FIG. 17 partially in section in a structure.

[0033]FIG. 19 is a side elevational view of a fourth embodiment of a captive screw according to the present invention, partially in section in a heat sink and a circuit board.

[0034]FIG. 20 is a side elevational view of the captive screw of FIG. 19 partially in section in a structure.

[0035]FIG. 21 is a perspective view of the captive screw of FIG. 19.

[0036]FIG. 22 is a side elevational view of the captive screw of FIG. 19.

[0037]FIG. 23 is a bottom view of the captive screw of FIG. 19.

[0038]FIG. 24 is a perspective view of the collar of the captive screw of FIG. 19.

[0039]FIG. 25 is a perspective view of the collar of the captive screw of FIG. 19.

DETAILED DESCRIPTION

[0040] The present invention provides a captive screw for securing a first structure such as a first panel or heat sink to a second structure such as a circuit board.

[0041] Referring now to the drawings in detail, wherein like reference numerals indicate like elements throughout the several views, there is shown in FIG. 1 a captive screw 10 according to the present invention. The captive screw 10 includes a ferrule 20, a screw 40 and a spring 50. The screw 40 (FIG. 2) includes a head 42 adapted to receive a driver (FIG. 3), a shank 44 adapted to pass through the ferrule 20, and a threaded portion 46 at the end of the shank 44 opposite the head 42. The 40 screw also includes a collar 48 formed on the shank 44 proximate the threaded portion 46.

[0042] The generally cylindrical ferrule 20 can have a plurality of knurls formed on its exterior cylindrical surface for securing the captive screw 10 in a preformed aperture 102 in a first structure 100 such as a heat sink by a press fit. The threaded portion 46 of the screw 40 is intended for securing the captive screw 10 in a preformed, threaded aperture 112 formed in second structure 110 such as a circuit board shown in FIG. 1 having a chip 5. The chip is shown on a sheet 6 of flexible material which absorbs vibrations. The captive screw 10 is positioned precisely relative to the second structure by virtue of the collar 48 formed on the shank 44 of the screw 40. The collar 48 limits the depth of penetration of the screw 40 into a threaded insert 7 which has been inserted from the bottom of the circuited board 112. Alternatively, the threaded portion of the captive screw can be screwed directly into a preformed, threaded aperture in a second structure. By coming into contact with the threads of threaded insert 7, the screw 40 is being rotationally driven into the second structure.

[0043] As can be seen in the FIGS. 1, 3 and 4, the ferrule 20 is formed with an annular ring section 24. The captive screw 10 is assembled by placing the spring 50 on the screw 40 and then passing the screw 40 through the ferrule 20. The annular ring 24 on the interior surface of the ferrule is large enough to permit the threads 46 of the screw 40 to pass through but not large enough to permit the spring 50 to pass through. When the ferrule 20 is formed during assembly, pressure can be applied uniformly on the generally cylindrical portion of the ferrule and plastic deformation can form the annular ring 24. Prior to the pressure being applied to the ferrule the ferrule can have an annular circumferential groove which allows for the formation of the ring section. The annular ring section 24 reduces the size of the opening in the ferrule so that the resultant opening is slightly larger than the diameter of the screw shaft 44 but smaller than both the screw head diameter and the diameter of the collar, thus captivating the screw 40 on the ferrule 20 in between the screw head 42 and the collar 48. The collar can be rolled onto the screw after the screw has been inserted into the ferrule. After installation of the ferrule into the first structure 100 the end of the ferrule can be flared.

[0044] The ferrule 20 also has an annular lip 28 or stop formed on the exterior cylindrical surface proximate the top of the ferrule 20 for limiting the penetration of the ferrule 20 in the preformed aperture 102 in the first structure 100. When the captive screw of 10 the present invention is installed, the collar 48 precisely limits the vertical position of the screw 40 above the second structure 110, while the first structure 100 floats above the second structure 110, while being urged towards the second structure 110 by the spring 50 of the captive screw 10. For example, FIG. 6 is a perspective view of the captive screw of the first embodiment of the invention showing the position of the screw when the screw is in a fastened position. FIG. 8 is a perspective view of the captive screw of the first embodiment of the invention showing the position of the screw when the screw is in a fastened position.

[0045] In the second embodiment of the invention as shown in FIG. 16, the ferrule 120 also has an annular lip 38 or stop formed on the exterior cylindrical surface proximate the top of the ferrule 120 for limiting the penetration of the ferrule 120 in the preformed aperture 102 in the first structure, as well as an annular circumferential groove 30 formed in the exterior cylindrical surface of the ferrule immediately adjacent and below the annular lip 38, for receiving the plastic flow of material 104 when the ferrule 120 is pressed into the preformed aperture. When the captive screw of 310 the present invention is installed, the collar 48 precisely limits the vertical position of the screw 40 above the second structure (not shown), while the first structure 100 floats above the second structure, while being urged towards the second structure by the spring 50 of the captive screw 310. The captive screw shown in FIG. 1 can be pressed into the underside of the first structure from the bottom. Alternately, the captive screw shown could be installed from the top when groove 30 is placed near the bottom of the first structure 30. Also, the collar of the second embodiment can be shaped as part of the screw after the screw has been inserted in the ferrule.

[0046] In the third embodiment of the invention as shown in FIGS. 17 and 18, the ferrule 220 also has a first annular lip 58 or stop formed on the exterior cylindrical surface proximate the top of the ferrule 220 for limiting the penetration of the ferrule 220 in the preformed aperture 102 in the first structure, as well as an annular circumferential groove 130 formed in the exterior cylindrical surface of the ferrule immediately adjacent and below the first annular lip 58, for receiving the plastic flow of material 104 when the ferrule 220 is pressed into the preformed aperture. A second annular lip 68 is shown at the opposite end of the ferrule. When the captive screw of 210 the present invention is installed, the collar 48 precisely limits the vertical position of the screw 40 above the second structure (not shown), while the first structure 100 floats above the second structure, while being urged towards the second structure by the spring 50 of the captive screw 210. In this embodiment of the present invention the collar can be pressed on.

[0047] In the fourth embodiment, FIGS. 19, 20, 21 and 22 show a captive screw in first structure 100 having a collar 148 which has outwardly extending legs and a base portion. The collar has outwardly extending legs 248 and the base portion 348. The ends 448 of the legs 248 extend outwardly and define a collar cross section. Since the legs are flexible, the ends of the legs deform elastically and permit the collar to pass through a first structure when being passed through an aperture in a structure and the aperture has a smaller cross section than that defined by the ends of the legs. The arrangement shown in FIG. 19 is one way in which to install the collar 48 into a second structure 110. The base portion 348 of the collar limits the penetration of the screw into the second structure. In the embodiment shown in FIG. 20 the ends 448 of the legs contact the underside of the first structure. The collar can be in the shape of a crown and the collar can be threaded on, crimped on or pressed on, etc. The legs are also dimensioned and configured to not allow the spring to pass over the legs.

[0048] The collar shown and described in the first three embodiments can be rolled on pressed on or screwed on.

[0049] The screw in the above embodiments includes a generally cylindrical head 42 having a central recess 34 formed therein and adapted to receive a driver. While a Phillip-type driver recess is illustrated, other types of driver recesses, such as slotted, Torx®, hexagonal, and the like, and combinations thereof (e.g. slot-Torx), can also be used. A generally cylindrical upper section extends coaxially downward from the head 42.

[0050] Preferably, the screw, and ferrule are formed from a suitable metallic material such as aluminum.

[0051] While the ferrule of the present invention is adapted to be press fit into the first structure, other means of mounting the captive screw on the structure can be employed, including conventional installation methods known as “flare-in,” “floating,” “p.c. board,” and “snap-in” installation methods.

[0052] Various other modifications can be made in the details of the various embodiments of the apparatus of the present invention, all within the scope and spirit of the invention and defined by the appended claims. 

We claim:
 1. A captive screw comprising: a ferrule having an inner annular ring on an interior surface of the ferrule; a screw having a head, a shank adapted to pass through the ferrule, a threaded portion at the end of the shank opposite the head, and a collar formed on the shank proximate the threaded portion; and a spring extending on the shank of the screw between the head and the ferrule; wherein the screw is captivated on the ferrule between the head and the collar, and the inner ring is dimensioned such that the collar does not permit movement of the collar past the inner annular ring but permits the passing through of the threads of the screw through the collar.
 2. The captive screw according to claim 1 wherein the ferrule has a generally cylindrical exterior surface having a plurality of knurls for securing the captive screw in a preformed aperture in a first structure.
 3. The captive screw according to claim 1 wherein the inner annular ring forms a generally circular opening in the ferrule.
 4. The captive screw according to claim 2 wherein the ferrule has an annular lip formed on the exterior cylindrical surface proximate the top of the ferrule for limiting the penetration of the ferrule in the preformed aperture in the first structure.
 5. The captive screw according to claim 2 wherein the ferrule has an annular circumferential groove formed in the exterior cylindrical surface of the ferrule immediately adjacent and below the annular lip for receiving the plastic flow of material when the ferrule is pressed into the preformed aperture.
 6. A captive screw comprising: a ferrule having a first and a second opposed ends and an inner annular ring on an interior surface of the ferrule, said ferrule having an annular lip at one of said first and second opposed ends and an annular circumferential exterior groove adjacent said annular a screw having a head, a shank adapted to pass through the ferrule, a threaded portion at one end of the shank opposite the head, and a collar formed on the shank proximate the threaded portion; and a spring extending on the shank of the screw between the head and the ferrule; wherein the screw is captivated on the ferrule between the head and the collar, and the inner annular ring is dimensioned such that the collar does not permit movement of the collar past the inner annular ring but permits the passing through of the threads of the screw through the collar.
 7. The captive screw according to claim 6 wherein the annular lip and the circumferential exterior groove are dimensioned and configured for pressing the ferrule into a panel and securing the captive screw in a preformed aperture in a first structure.
 8. The captive screw according to claim 6 wherein the end of the ferrule at which the annular lip is located faces the screw head.
 9. The captive screw according to claim 6 wherein the end of the ferrule at which the annular lip is located faces the threaded portion of the screw.
 10. The captive screw according to claim 6 wherein the inner annular ring forms a generally circular opening.
 11. The captive screw according to claim 6 wherein the inner ring is formed by plastic flow of the ferrule during assembly of the captive screw.
 12. A captive screw comprising: a ferrule having a first and a second opposed ends, said ferrule having: a first annular lip at one of said first and second opposed ends; an annular circumferential exterior groove adjacent said first annular lip; and a second annular lip adjacent to the annular circumferential exterior groove and located across the annular circumferential exterior groove with respect to the first annular lip; a screw having a head, a shank adapted to pass through the ferrule, a threaded portion at one end of the shank opposite the head, and a collar formed on the shank proximate the threaded portion; and a spring extending on the shank of the screw between the head and the ferrule; wherein the screw is captivated on the ferrule between the head and the collar, and the ferrule is dimensioned such that the collar does not permit movement of the collar past the ferrule but permits the passing through of the threads of the screw through the collar.
 13. The captive screw according to claim 12 wherein the ferrule has a generally circular interior surface.
 14. The captive screw according to claim 12 wherein the annular lip and the circumferential exterior groove are dimensioned and configured for pressing the ferrule into a panel and securing the captive screw in a preformed aperture in a first structure.
 15. The captive screw according to claim 12 wherein the first annular lip extends outwardly away from the ferrule farther than the second annular lip.
 16. The captive screw of claim 12 in combination with a heat sink.
 17. A captive screw comprising: a screw having a head, a shank adapted to pass through the ferrule, a threaded portion at one end of the shank opposite the head, and a collar having a plurality of outwardly extending legs formed on the shank proximate the threaded portion; and a spring extending on the shank of the screw between the head and the legs of the collar; wherein said collar is dimensioned and configured so as not to permit the passing of the spring past the collar thereby captivating the screw.
 18. The captive screw of claim 17 wherein the outwardly extending legs are substantially flexible thereby permitting the legs to fit through an aperture in a structure which has a cross section larger than a cross section of the collar defined by the ends of the legs when the legs of the collar are in an unflexed state.
 19. The captive screw of claim 17 wherein the collar is in the shape of a crown.
 20. The captive screw of claim 16 in combination with a heat sink. 